Always search Ansible's stock modules

Without this commit, configuring the `library` search path in
ansible.cfg means Ansible will not find its standard modules unless
you explicitly include Ansible's default module directory in that
path.  This is inconsistent with how all plug-ins work, since the
standard Ansible plug-ins are always available, even if you set the
`*_plugins` variables in ansible.cfg.  Having to explicitly provide
Ansible's default module directory is also annoying if you want to use
the ansible.cfg across systems where Ansible is installed in different
locations.

With this commit, Ansible will always search its standard modules.
The standard modules are always searched last, after any directory
named "library" in your CWD and after any directories specified in
ansible.cfg.

This is an attempt to slim down my previous request #3392.  Just to
reiterate the above, this change does not prevent you from
modifying and overriding the standard modules.